The short version
Key points
- An inverter’s kilowatt rating determines how much power it can continuously supply to the home.
- A battery may store a large amount of energy but still have a lower maximum output than the inverter.
- Three-phase systems can distribute inverter output across phases, which may limit the power available to a single appliance or phase.
- A larger inverter may require additional solar panels to use its capacity effectively.
- AC coupling can allow an existing solar system to be retrofitted with a battery without replacing the existing solar inverter.
Why inverter size matters
The inverter converts the direct current from solar panels and batteries into the alternating current used by household appliances. Its kilowatt rating sets the maximum continuous power it can provide. If a home tries to draw more than that rating, the inverter may shut down, reduce its output or allow additional power to come from the grid, depending on the system.
The inverter also limits the rate at which a battery can charge or discharge. For example, a 3 kW inverter cannot charge a battery at more than 3 kW, even if the solar panels are producing 6 kW. The aim is to choose an inverter that covers the home’s likely peak demand, with some headroom, without paying for capacity that is rarely used.
Sizing for household demand
A basic approach for a single-phase home is to add the power requirements of appliances that could operate at the same time. Pat gives an example involving a 2 kW oven, a 3 kW electric cooktop, a 2 kW kettle and a 2 kW heater, producing a combined demand of about 9 kW. Adding a 20 per cent buffer takes the requirement to approximately 11 kW, beyond the capacity of a 10 kW inverter.
In practice, many households stagger their larger loads. However, homes with multiple air conditioners, electric underfloor heating, pool pumps or an electric vehicle charger may need to consider an inverter of at least 10 kW. The battery must also be able to provide enough instantaneous power to support the inverter. A system with a 15 kW inverter and batteries limited to 7.5 kW of output will not deliver 15 kW from the battery.
Three-phase considerations
Three-phase homes require additional planning because a three-phase inverter will typically divide its output across the three phases. A 9 kW inverter could provide up to 3 kW per phase. If a 5 kW ducted air conditioner is connected to one phase, the inverter may not be able to supply all of its demand on that phase, even if spare capacity exists on the other two.
Smart meters can account for imports and exports across phases, according to the transcript, but phase balancing can become more important during a blackout. Some inverters support unbalanced output, although standards such as AS/NZS 4777 limit how much imbalance is permitted. The inverter’s specifications and its blackout operation should therefore be checked with the installer.
Solar panels, batteries and local rules
A larger inverter needs enough solar generation or battery output to make use of its capacity. Pat notes that a 10 kW inverter paired with 6.6 kW of panels will seldom produce more than the panels can generate. Additional panels can help extend production into the morning and afternoon while supplying household loads and charging the battery.
Existing solar panels do not always need to be replaced when adding a battery. AC-coupled batteries use their own inverter and connect on the alternating-current side, which can simplify a retrofit. A DC-coupled hybrid inverter requires the existing panels to meet its voltage and current limits, and the panels may need to be rearranged, added to or replaced.
The transcript also highlights that network rules vary. It states that single-phase homes can generally have up to 10 kVA of inverter capacity with a 5 kW export limit, while multi-phase homes may generally reach 15 kW total, subject to phase limits and approvals. In Western Australia, the transcript notes that three-phase systems can reach 15 kW total, with a 1.5 kW export limit on each phase. These requirements should be confirmed with the relevant network and installer.
Pat’s system decision
Pat initially planned to use a 5 kW inverter with a 25 kWh battery and 6.6 kW of panels in his three-phase home. His gas water heater, gas stove and 10 amp wall-plug oven reduced his expected electrical demand, while his stated daytime and evening tariffs also influenced the calculation.
He ultimately decided to double both the panel capacity and inverter size. The change provided more headroom for possible future additions, including an electric cooktop and water heater, and allowed more battery charging from solar rather than depending on a tariff that might not remain available.
TechManPat’s conclusion
In my view, choosing an inverter is a balance between the budget, current and future household demand, solar generation, battery output and local network rules. I would especially check the battery’s maximum kilowatt output, because storage capacity alone does not show how much power the system can deliver during normal use or a blackout.
This knowledge-centre summary is based on the linked TechManPat video and reflects the information available when it was published. Check current pricing, availability and policies before acting.
